1. Field of the Invention
The present invention relates to a technique for detecting output current in a current resonance type switching power supply circuit.
2. Description of Related Art
In some cases, detection of output current is required in a DC-DC converter for the purpose of overcurrent protection and current mode control. Here, according to the current mode control, not only an output voltage, but also a current detection signal of an output inductor is fed back to perform switching control, thereby improving the power factor, enhancing the line regulation, etc.
Furthermore, detection of output current is also required in a multi-phase type power supply circuit to adjust current balance of respective converters in some cases. According to the multi-phase type, a plurality of DC-DC converters are operated in parallel to one another and respective outputs thereof are added to one another while shifting the output phase every converter, thereby obtaining a stable output having a low ripple.
The following disclosures are known as a system for detecting the output current in the DC-DC converter. For example, Japanese Patent Application Laid-Open Publications No. 2004-297943 discloses a system in which a current detecting resistor R is connected to an output current pathway and a voltage occurring across this resistor is detected as shown in FIG. 8A. Furthermore, Japanese Patent No. 3,254,199 discloses a system in which a resistor R2 and a capacitor C are connected in parallel with an output coil Lo and output current is detected on the basis of the voltage of the capacitor C by using the resistance component R1 of the output coil Lo as shown in FIG. 8B. Still furthermore, Japanese Patent Application Laid-Open Publications No. 2003-284328 discloses a system in which output current is detected on the basis of the drain-source voltage under ON-operation of the transistor SW2 by using the ON-resistance of a synchronous rectification transistor SW2 as shown in FIG. 8C.
Furthermore, Japanese Patent Application Laid-Open Publications No. 7-236271 discloses a system in which the output current is detected from the peak value of the resonance voltage of an LC resonance circuit, because there is a correlation between the peak voltage of a resonance voltage and output current in a voltage resonance type switching power supply device.
The resonance type switching power supply circuit has an advantage that a switching loss and a switching noise are reduced, and comparing the voltage resonance type and the current resonance type, they have their peculiar advantages respectively. Therefore, it is required in some cases to detect the output current and execute various kinds of control based on the detection of the output current while adopting the current resonance type switching power supply circuit.
The output current can be also detected by applying the systems of FIGS. 8A to 8C in the current resonance type switching power supply circuit. However, the system of FIG. 8A has a disadvantage that a large loss occurs in the current detecting resistor R. Furthermore, the system of FIG. 8B has a disadvantage that the number of circuit elements is increased for detecting current and also has a problem that it is impossible to detect accurate current unless a temperature compensation circuit is added because the resistance component R1 of the coil Lo has a relatively large temperature characteristic.
The system of FIG. 8C has a problem that a temperature compensation circuit is required to perform accurate current detection because the ON-resistance of FET has a temperature characteristic, and further the systems of FIGS. 8B and 8C have a problem that a very high-precision detecting circuit is required for detecting a small voltage generated by small resistance.
Unlike the voltage resonance type circuit disclosed in Japanese Patent Application Laid-Open Publications No. 7-236271, the current resonance type switching power supply circuit does not have such a simple correlation that the output current amount is proportional to the peak value of the resonance voltage, and thus it is impossible to detect the output current by applying the technique disclosed in Japanese Patent Application Laid-Open Publications No. 7-236271.